Plastic working (e.g., bending, twisting) on a spring steel material to form the spring steel material into a spring shape generates a machining strain in the spring steel material. Due to an adverse effect of the machining strain on spring characteristics (e.g., durability, setting resistance), a heat treatment (a so-called “low temperature annealing treatment”) for eliminating the machining strain of the spring steel material is executed after forming the spring steel material into a spring shape (Japan Society of Spring Engineers, “Spring,” 4th edition, P. 463 to 466, Maruzen Co., Ltd.). This heat treatment normally uses a heating furnace such as a hot-blast stove or an infrared heating furnace. In a case where the heat treatment is executed using such heating furnace, the spring steel material formed into the spring shape is introduced into the heating furnace from its one end. The spring steel material introduced in the heating furnace is heated while being conveyed toward the other end of the heating furnace, and carried out from the other end of the heating furnace to the outside of the heating furnace. The spring steel material is subjected to the heat treatment in this manner, and consequently the machining strain is eliminated from the spring steel material. It should be noted in this heat treatment that the treatment temperature is generally set at 380 to 430° C. and the treatment time at 20 to 60 minutes.